fix moves preview

app/src/main/java/chess/controller/CommandExecutor.java

@@ -1,6 +1,7 @@
 package chess.controller;
 
 import chess.controller.Command.CommandResult;
+import chess.controller.commands.UndoCommand;
 import chess.model.Game;
 import chess.model.Game.GameStatus;
 
@@ -21,7 +22,7 @@ public class CommandExecutor {
 		CommandResult result = command.execute(this.game, this.outputSystem);
 
 		// non player commands are not supposed to return move result
-		assert result != CommandResult.Moved || command instanceof PlayerCommand;
+		assert result != CommandResult.Moved || command instanceof PlayerCommand || command instanceof UndoCommand;
 
 		processResult(command, result);
 

app/src/main/java/chess/view/simplerender/Window.java

@@ -138,19 +138,7 @@ public class Window extends JFrame implements OutputSystem {
 		}
 	}
 
-	private void clearPreviews() {
-		for (int y = 0; y < 8; y++) {
-			for (int x = 0; x < 8; x++) {
-				JLabel cell = this.cells[x][y];
-				cell.paintComponents(getGraphics());
-			}
-		}
-	}
-
 	private boolean previewMoves(int x, int y) {
-
-		clearPreviews();
-
 		GetAllowedMovesCommand movesCommand = new GetAllowedMovesCommand(new Coordinate(x, y));
 		if (sendCommand(movesCommand) == CommandResult.NotAllowed)
 			return false;
@@ -180,13 +168,13 @@ public class Window extends JFrame implements OutputSystem {
 			for (int x = 0; x < 8; x++) {
 				JLabel cell = this.cells[x][y];
 				cell.setBackground(getCellColor(x, y));
+				cell.paint(cell.getGraphics());
 			}
 		}
 	}
 
 	private void onCellClicked(int x, int y) {
 		clearMoves();
-		clearPreviews();
 		if (this.lastClick == null) {
 			if (isCellEmpty(x, y))
 				return;